A conventional airplane detection system (U.S. patent application Ser. No. 07-249173, now U.S. Pat. No. 5,027,114) measures a change caused by an airplane in the inductance of an electromagnetic loop coil, to thereby detect the airplane moving along a runway or a taxiway. Another prior art (Japanese Patent Application No. 3-349628) discloses an electromagnetic wave sensor system employing a wave transmitter and a wave receiver arranged on each side of a runway, to transmit and receive electromagnetic waves (microwaves). When the electromagnetic waves are blocked, the system determines that there is an airplane.
The former system with the electromagnetic loop coil must lay the loop coil under a runway. This may damage the runway. The latter electromagnetic wave sensor system (an energy transmission type sensor system) for detecting an airplane also has problems mentioned below.
This electromagnetic wave sensor system arranges the wave transmitter and wave receiver on opposite sides of a runway. When the fuselage of an airplane continuously blocks electromagnetic waves, the system may correctly detect the airplane. The wave transmitter and wave receiver, however, must be installed on each side of the runway at heights that never interfere with the wings, etc., of the airplane. The wave transmitter and wave receiver are not allowed to be arranged at levels where the fuselage blocks the electromagnetic waves. The electromagnetic waves, therefore, are directed toward wheels under the fuselage, to detect the airplane. In this case, the electromagnetic waves received by the wave receiver intermittently attenuate because they are intermittently blocked by the nose and main wheels of the airplane. Consequently, this detection system never provides a continuous detection signal. It is required to provide an airplane detected signal continuously from the detection of the nose wheel (nose gear) of the airplane to the detection of the main wheels (main gears) thereof.
A large airplane usually has a nose gear and two main gears. When the airplane is seen widthwise, the two main gears overlap. Accordingly, an attenuation in electromagnetic wave energy blocked by the main gears is temporally longer than art attenuation in that blocked by the nose gear. Based on this fact, a signal may be generated when the nose gear causes an attenuation in electromagnetic waves and may be stopped when it is judged the main gears cause a longer attenuation in the electromagnetic waves. According to this technique, the second attenuation will not always be longer than the first attenuation, if the moving speed of the airplane is changed, or if the airplane is temporarily stopped with the nose gear blocking the electromagnetic waves.
It may be possible to continuously provide the detection signal for a set period after detecting the nose gear. It is very difficult, however, to determine the duration of the period because the airplane may be stopped after the nose gear is detected.
In this way, the conventional energy transmission type electromagnetic wave sensor system detects an airplane only under a specific condition that the airplane moves at a constant speed. If the moving speed of the airplane is changed while the airplane is passing through the sensor system, or if the airplane is stopped to straddle electromagnetic waves, this system hardly detects the airplane.
Accordingly, an object of the present invention is to provide an airplane detection system employing a combination of an energy transmission type electromagnetic wave sensor and a radar type electromagnetic wave sensor. This system never damages a runway and surely detects an airplane even if the airplane is stopped halfway.